Many advanced electronic and opto-electronic integrated circuits are based on compound semiconductors such as the III-V semiconductors. Gallium arsenide (GaAs) is the basis of a fairly well developed technology; indium phosphide (InP) and related materials are not so well developed but have received much attention, especially for active opto-electronic devices, such as lasers and optical modulators, operating in the 1.55 .mu.m band of optical wavelengths which is of great interest for integration with silica optical fibers. Indium gallium arsenide (InGaAs) is often considered to be an InP-based material because alloys with InP can be made that have little change in lattice constant while providing bandgap control of the commercially important part of the optical spectrum around 1550 nm.
A fundamental advantage of III-V semiconductors is that modern film growth techniques, such as organo-metallic chemical vapor deposition (OMCVD) and molecular beam epitaxy (MBE), enable the epitaxial growth of thin films with nearly arbitrary III-V compositions, assuming equality of Group-III cations and Group-V anions, thus allowing many important semiconductor characteristics such as electronic bandgap to be freely engineered. Similar freedom is available with II-VI semiconductors. An important structure that is so enabled is the single or multiple quantum-well (MQW) structure much used for lasers and modulators, an example of which is illustrated schematically in FIG. 1 with the horizontal axis representing the epitaxial growth direction and the vertical axis representing the electronic bandgap for the different materials. For example, an electronic diode structure includes an n-type InP layer 10 and a p-type InP layer 12 sandwiching an undoped active layer 14 comprising alternating thin layers of InGaAs wells 16 and InGaAsP barriers 18. The wells and barriers 16 and 18 are thin enough, usually less than 10 nm, that one or more quantum mechanical valence states 20 and conduction states 22 form within the wells 16. The number of quantum wells may be one or more.
The effective bandgap between the valence and conduction states 20, 22 within the wells 16 depend both upon the well composition and the thickness of the well. Although the compositions are generally chosen to be lattice matched to the InP substrate, a controlled amount of strain can be introduced into the wells and barriers to further control the electronic band structure. The result is an active layer 14 having a high density of narrow electronic states, assuming the wells 14 have been well fabricated, with the effective bandgap that determines optical characteristics being easily varied. In a typical opto-electronic device, electrical leads are connected to the two InP layers 10, 12 and an unillustrated optical waveguiding structure is formed along the active layer 14 in the directions perpendicular to the illustrated z-direction so as to confine a major portion of the optical wave within the active layer 14 to there interact with the electrically controlled carriers.
However, the process of forming the optical confinement structure tends to degrade the multi quantum-well structure. A typical though simplified buried heterostructure MQW laser is illustrated in cross section in FIG. 2. The vertical planar structure of FIG. 1 is grown and then patterned and etched so as to form a ridge extending along the y-direction and having a finite width along the x-direction of the active layer 14 including the multiple quantum wells. Thereafter, a semi-insulating InP 24 is epitaxially regrown around the ridge to reduce the contrast of the refractive index of the active layer 14 relative to that of the surrounding material and to confine the biasing current to the active layer 14. The structure shown in FIG. 2 is simplified for ease of presentation. More layers may be included to, for example, better confine the light to the core, but the illustrated structure is sufficient to explain the effect of the invention. More realistic structures for buried heterostructure lasers are described by Odagawa et al. in "High-Speed Operation of Strained InGaAs/InGaAsP MQW Lasers Under Zero-Bias Condition," IEEE Journal of Quantum Electronics, vol. 29, 1993, pp. 1682-1686 and by Aoki et al. in "Monolithic integration of DFB lasers and electroabsorption modulators using in-plane quantum energy control of MQW structures, International Journal of High Speed Electronics and Systems, vol. 5, 1994, pp. 67-90.
The regrowth of the fairly thick semi-insulating layer 24 imposes a large thermal budget on the already fabricated quantum wells. Even the after grown upper cladding layer 12 incurs a significant thermal budget. OMCVD of these materials is typically done between 625.degree. and 650.degree. C. so that temperatures between 600.degree. and 700.degree. C. should be anticipated. Even higher temperatures may be required for explicit annealing. The thermal treatment of the quantum wells in these temperature ranges has been generally observed to shift the bandgap between the well states to the blue. That is, the effective bandgap of the well states anneal to larger bandgap energy. Also, the potential wells tend to lose their rectangular shape. The structure described by Aoki et al., ibid., includes both lasers and modulators having different well thickness and involves two regrowths, one for the upper, p-type InP layer and another for the semi-insulating InP. Thus significant blue shifting is expected, but the amount of blue shift will differ between the laser and modulator because of the differing well thicknesses.
The size of the blue shift has been observed to shift the photoluminescence peak by about 10 to 40 nm at devices designed for 1550 nm. However, the shift varies across a wafer and from wafer to wafer. A shift in the wavelength peak of the photoluminescent emission presents a problem in fabricating lasers and modulators since, for example, optimum performance in distributed feedback lasers requires the wavelength of the gain peak to match the grating pitch. In the case of modulators, a variation of the blue shift between different ones of the multiple quantum wells will produce a less steep change of absorption with wavelength, thereby degrading the modulator performance.
Several suggestions have been made to reduce the blue shift. One entails the use of substrates with high dislocation densities, the dislocation pipes acting as gettering sites for the species, speculated to be phosphorus interstitials, responsible for the blue shift. This solution is not attractive because heavily dislocated substrates introduce concerns about the reliability of devices formed on them.
Another suggestion involves the use of strained quantum wells in which both the wells and the barriers have the same compositional ratio As/P of the Group-V components, thereby avoiding any effect from the mobile phosphorus. Although this solution seems effective, it restricts the device design.
Several groups have reported their understanding of the mechanism for blue shift in Proceedings of Fifth International Conference on Indium Phosphide and Related Materials, Apr. 19-22, 1993, Paris, France (IEEE Catalog #93CH3276-3). See Glew, "Interdiffusion of InGaAs/InGaAsP quantum wells," ibid., pp. 29-32; Gillin et al., "Group V interdiffusion in In.sub.0.66 Ga.sub.0.33 As/In.sub.0.66 Ga.sub.0.33 As.sub.0.7 P.sub.0.3 quantum well structures," ibid., pp. 33-35; Camassel et al., "Experimental investigation of the thermal stability of strained InGaAs/InGaAsP MQWs," ibid., pp. 36-39; and Vettese et al., "An investigation into the effects of thermal annealing on long wavelength InGaAs/InGaAsP multi-quantum well lasers," Ibid., pp. 40-44. Although diffusion of phosphorus is a recurring theme, there is no agreement on the responsible mechanism.
Accordingly, a more reliable and less restrictive method is desired for controlling and reducing the blue shift in InP-based and related quaternary quantum-well structures.